Heavy metals, radionuclides, and halogenated hydrocarbons constitute major contaminants in the clean-up and decontamination of aqueous waste. Virtually every site or aqueous stream that requires remediation contains contaminants from at least one of these three groups.
For example, many of the Superfund sites administered by the U.S. Environmental Protection Agency involve heavy metals such as lead, mercury, or chromium from factories previously occupying these sites. The U.S. Department of Energy's waste sites at Hanford, Washington, and at the Savannah River, South Carolina contain vast amounts of aqueous waste with significant amounts of radionuclides such as strontium-90, cesium-137, and technetium-99 are present. Halogenated hydrocarbons such as perchlorethylene (PCE) and trichlorethylene (TCE) are common industrial solvents still in wide use today, e.g. drycleaners. These halogenated hydrocarbons are highly toxic and are likely carcinogens with safe drinking water limits of less than 10 parts per billion. An accidental spill of these chlorinated hydrocarbons could cause serious hazardous risks to municipal water supplies.
Previously, removal of these types of contaminants required very expensive treatment procedures. These procedures primarily use pump-and-treat methods wherein water was pumped out of the soil, treated, and then pumped back into the ground. In addition, expensive reagents such as ethylene diaminetraacetic acid (EDTA) or expensive materials such as ultrafilters are required for these treatments.
In the case of halogenated hydrocarbon contaminants, a recent development which has garnered strong interest is the use of iron filings or powders, or iron sulfide(pyrite), for in-situ treatment of contaminated groundwater streams. The iron (or the pyrite) has been found to degrade hydrocarbons by reductive elimination and/or reductive dehalogenation of the parent compound and its daughter products. In use, the iron (or pyrite) is placed in a trench or a well that is located perpendicular to the flow of the groundwater, with the intent of leaving the reagent permanently in place. While this method obviates the necessity for maintainence, and is much less expensive than pump-and-treat methods, the relatively low reactivity of the iron or pyrite requires large amounts of reagent to achieve complete decontamination. Thus, if iron filings were used, a large emplacement would be required, entailing large initial capital costs.
In the case of radionuclides, treatment of radionuclide contamination entails special considerations. Radionuclide contaminants invariably involve a national government, have little or no margin for losses to the environment, and must be recovered in a form suitable for handling and further processing for long-term storage.
A radionuclide that is of special concern is technetium. Technetium is an artificial, radioactive element that is a by-product of both weapons dedicated and energy generating nuclear fission plants. It presents a particular difficulty in that it commonly forms the pertechnetate anion (TcO.sub.4.sup.-). It is of global concern as it is not only a contaminant at the Department of Energy's Hanford, Savannah River, and Oak Ridge sites, but is also present at the Chernobyl disaster site, and continues to be produced by fission plants throughout the world.
Technetium (Tc) removal is a high priority need. Waste separation, pretreatment, and vitrification processes for technetium removal involve separation into solid high level waste (HLW) and low level waste (LLW) fractions. The latter contains most of the Tc, predominantly in the +7 oxidation state as the pertechnetate ion (TcO.sub.4.sup.-). Vitrification of these waste fractions is problematic because Tc.sup.7+ compounds are volatile at high temperatures, and the presence of large quantities of nitrate and nitrate in the LLW ensures that the melts remain oxidizing.
Methods of extracting Tc from the aqueous media such as LLW prior to concentration and vitrification are urgently needed to address this volatility problem, as well as to reduce the total volume of vitrified waste. These needs are driven by both safety and cost considerations.
While other radionuclides such as strontium and cesium are present in low-level waste form as cations and can be sorbed onto clays and micas, or, under some conditions, precipitated with tetraphenylborate, the pertechnetate ion carries a negative charge and exhibits little or none of the usual sorptive precipitation behavior of metal ions. Thus, it is very difficult to contain and process technetium with any previously existing technology.
A need therefore exists for remediation of radionuclides, metal contaminants and halogenated hydrocarbons in groundwater. A particular need exists for remediation of radionuclides such as technetium and heavy metals such as such as chromium, lead, mercury and titanium in groundwater.